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<h1>MEP27: Decouple pyplot from backends<a class="headerlink" href="#mep27-decouple-pyplot-from-backends" title="Link to this heading">#</a></h1>
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<p>Main PR (including GTK3):</p>
<ul class="simple">
<li><p><a class="github reference external" href="https://github.com/matplotlib/matplotlib/pull/4143">matplotlib/matplotlib#4143</a></p></li>
<p>Backend specific branch diffs:</p>
<ul class="simple">
<li><p><a class="github reference external" href="https://github.com/OceanWolf/matplotlib/compare/backend-refactor...OceanWolf:backend-refactor-tkagg">OceanWolf/matplotlib</a></p></li>
<li><p><a class="github reference external" href="https://github.com/OceanWolf/matplotlib/compare/backend-refactor...OceanWolf:backend-refactor-qt">OceanWolf/matplotlib</a></p></li>
<li><p><a class="github reference external" href="https://github.com/OceanWolf/matplotlib/compare/backend-refactor...backend-refactor-wx">OceanWolf/matplotlib</a></p></li>
<section id="abstract">
<h2><a class="toc-backref" href="#id3" role="doc-backlink">Abstract</a><a class="headerlink" href="#abstract" title="Link to this heading">#</a></h2>
<p>This MEP refactors the backends to give a more structured and
consistent API, removing generic code and consolidate existing code.
To do this we propose splitting:</p>
<ol class="arabic simple">
<li><p><code class="docutils literal notranslate"><span class="pre">FigureManagerBase</span></code> and its derived classes into the core
functionality class <code class="docutils literal notranslate"><span class="pre">FigureManager</span></code> and a backend specific class
<code class="docutils literal notranslate"><span class="pre">WindowBase</span></code> and</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">ShowBase</span></code> and its derived classes into <code class="docutils literal notranslate"><span class="pre">Gcf.show_all</span></code> and <code class="docutils literal notranslate"><span class="pre">MainLoopBase</span></code>.</p></li>
<section id="detailed-description">
<h2><a class="toc-backref" href="#id4" role="doc-backlink">Detailed description</a><a class="headerlink" href="#detailed-description" title="Link to this heading">#</a></h2>
<p>This MEP aims to consolidate the backends API into one single uniform
API, removing generic code out of the backend (which includes
<code class="docutils literal notranslate"><span class="pre">_pylab_helpers</span></code> and <code class="docutils literal notranslate"><span class="pre">Gcf</span></code>), and push code to a more appropriate
level in matplotlib.  With this we automatically remove
inconsistencies that appear in the backends, such as
<code class="docutils literal notranslate"><span class="pre">FigureManagerBase.resize(w,</span> <span class="pre">h)</span></code> which sometimes sets the canvas,
and other times set the entire window to the dimensions given,
depending on the backend.</p>
<p>Two main places for generic code appear in the classes derived from
<code class="docutils literal notranslate"><span class="pre">FigureManagerBase</span></code> and <code class="docutils literal notranslate"><span class="pre">ShowBase</span></code>.</p>
<ol class="arabic simple">
<li><p><code class="docutils literal notranslate"><span class="pre">FigureManagerBase</span></code> has <strong>three</strong> jobs at the moment:</p>
<ol class="arabic simple">
<li><p>The documentation describes it as a <em>Helper class for pyplot
mode, wraps everything up into a neat bundle</em></p></li>
<li><p>But it doesn't just wrap the canvas and toolbar, it also does
all of the windowing tasks itself.  The conflation of these two
tasks gets seen the best in the following line:
<code class="docutils literal notranslate"><span class="pre">self.set_window_title(&quot;Figure</span> <span class="pre">%d&quot;</span> <span class="pre">%</span> <span class="pre">num)</span></code> This combines
backend specific code <code class="docutils literal notranslate"><span class="pre">self.set_window_title(title)</span></code> with
matplotlib generic code <code class="docutils literal notranslate"><span class="pre">title</span> <span class="pre">=</span> <span class="pre">&quot;Figure</span> <span class="pre">%d&quot;</span> <span class="pre">%</span> <span class="pre">num</span></code>.</p></li>
<li><p>Currently the backend specific subclass of <code class="docutils literal notranslate"><span class="pre">FigureManager</span></code>
decides when to end the mainloop.  This also seems very wrong
as the figure should have no control over the other figures.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">ShowBase</span></code> has two jobs:</p>
<ol class="arabic simple">
<li><p>It has the job of going through all figure managers registered
in <code class="docutils literal notranslate"><span class="pre">_pylab_helpers.Gcf</span></code> and telling them to show themselves.</p></li>
<li><p>And secondly it has the job of performing the backend specific
<code class="docutils literal notranslate"><span class="pre">mainloop</span></code> to block the main programme and thus keep the
figures from dying.</p></li>
<section id="implementation">
<h2><a class="toc-backref" href="#id5" role="doc-backlink">Implementation</a><a class="headerlink" href="#implementation" title="Link to this heading">#</a></h2>
<p>The description of this MEP gives us most of the solution:</p>
<ol class="arabic simple">
<li><p>To remove the windowing aspect out of <code class="docutils literal notranslate"><span class="pre">FigureManagerBase</span></code> letting
it simply wrap this new class along with the other backend classes.
Create a new <code class="docutils literal notranslate"><span class="pre">WindowBase</span></code> class that can handle this
functionality, with pass-through methods (:arrow_right:) to
<code class="docutils literal notranslate"><span class="pre">WindowBase</span></code>.  Classes that subclass <code class="docutils literal notranslate"><span class="pre">WindowBase</span></code> should also
subclass the GUI specific window class to ensure backward
compatibility (<code class="docutils literal notranslate"><span class="pre">manager.window</span> <span class="pre">==</span> <span class="pre">manager.window</span></code>).</p></li>
<li><p>Refactor the mainloop of <code class="docutils literal notranslate"><span class="pre">ShowBase</span></code> into <code class="docutils literal notranslate"><span class="pre">MainLoopBase</span></code>, which
encapsulates the end of the loop as well.  We give an instance of
<code class="docutils literal notranslate"><span class="pre">MainLoop</span></code> to <code class="docutils literal notranslate"><span class="pre">FigureManager</span></code> as a key unlock the exit method
(requiring all keys returned before the loop can die).  Note this
opens the possibility for multiple backends to run concurrently.</p></li>
<li><p>Now that <code class="docutils literal notranslate"><span class="pre">FigureManagerBase</span></code> has no backend specifics in it, to
rename it to <code class="docutils literal notranslate"><span class="pre">FigureManager</span></code>, and move to a new file
<code class="docutils literal notranslate"><span class="pre">backend_managers.py</span></code> noting that:</p>
<ol class="arabic simple">
<li><p>This allows us to break up the conversion of backends into
separate PRs as we can keep the existing <code class="docutils literal notranslate"><span class="pre">FigureManagerBase</span></code>
class and its dependencies intact.</p></li>
<li><p>And this also anticipates MEP22 where the new
<code class="docutils literal notranslate"><span class="pre">NavigationBase</span></code> has morphed into a backend independent
<code class="docutils literal notranslate"><span class="pre">ToolManager</span></code>.</p></li>
<div class="pst-scrollable-table-container"><table class="table">
<tr class="row-odd"><th class="head"><p>FigureManagerBase(canvas, num)</p></th>
<th class="head"><p>FigureManager(figure, num)</p></th>
<th class="head"><p><code class="docutils literal notranslate"><span class="pre">WindowBase(title)</span></code></p></th>
<th class="head"><p>Notes</p></th>
<tr class="row-even"><td><p>show</p></td>
<td><p>show</p></td>
<tr class="row-odd"><td><p>destroy</p></td>
<td><p>calls destroy on all
components</p></td>
<td><p>destroy</p></td>
<tr class="row-even"><td><p>full_screen_toggle</p></td>
<td><p>handles logic</p></td>
<td><p>set_fullscreen</p></td>
<tr class="row-odd"><td><p>resize</p></td>
<td><p>resize</p></td>
<tr class="row-even"><td><p>key_press</p></td>
<td><p>key_press</p></td>
<tr class="row-odd"><td><p>get_window_title</p></td>
<td><p>get_window_title</p></td>
<tr class="row-even"><td><p>set_window_title</p></td>
<td><p>set_window_title</p></td>
<tr class="row-odd"><td></td>
<td><p>_get_toolbar</p></td>
<td><p>A common method to all
subclasses of FigureManagerBase</p></td>
<tr class="row-even"><td></td>
<td><p>set_default_size</p></td>
<tr class="row-odd"><td></td>
<td><p>add_element_to_window</p></td>
<div class="pst-scrollable-table-container"><table class="table">
<tr class="row-odd"><th class="head"><p>ShowBase</p></th>
<th class="head"><p>MainLoopBase</p></th>
<th class="head"><p>Notes</p></th>
<tr class="row-even"><td><p>mainloop</p></td>
<td><p>begin</p></td>
<tr class="row-odd"><td></td>
<td><p>end</p></td>
<td><p>Gets called
automagically
when no more
instances of
the subclass
exist</p></td>
<tr class="row-even"><td><p>__call__</p></td>
<td><p>Method moved
Gcf.show_all</p></td>
<section id="future-compatibility">
<h2><a class="toc-backref" href="#id6" role="doc-backlink">Future compatibility</a><a class="headerlink" href="#future-compatibility" title="Link to this heading">#</a></h2>
<p>As eluded to above when discussing MEP 22, this refactor makes it easy
to add in new generic features.  At the moment, MEP 22 has to make
ugly hacks to each class extending from <code class="docutils literal notranslate"><span class="pre">FigureManagerBase</span></code>.  With
this code, this only needs to get made in the single <code class="docutils literal notranslate"><span class="pre">FigureManager</span></code>
class.  This also makes the later deprecation of
<code class="docutils literal notranslate"><span class="pre">NavigationToolbar2</span></code> very straightforward, only needing to touch the
single <code class="docutils literal notranslate"><span class="pre">FigureManager</span></code> class</p>
<p>MEP 23 makes for another use case where this refactored code will come
in very handy.</p>
<section id="backward-compatibility">
<h2><a class="toc-backref" href="#id7" role="doc-backlink">Backward compatibility</a><a class="headerlink" href="#backward-compatibility" title="Link to this heading">#</a></h2>
<p>As we leave all backend code intact, only adding missing methods to
existing classes, this should work seamlessly for all use cases.  The
only difference will lie for backends that used
<code class="docutils literal notranslate"><span class="pre">FigureManager.resize</span></code> to resize the canvas and not the window, due
to the standardisation of the API.</p>
<p>I would envision that the classes made obsolete by this refactor get
deprecated and removed on the same timetable as
<code class="docutils literal notranslate"><span class="pre">NavigationToolbar2</span></code>, also note that the change in call signature to
the <code class="docutils literal notranslate"><span class="pre">FigureCanvasWx</span></code> constructor, while backward compatible, I think
the old (imho ugly style) signature should get deprecated and removed
in the same manner as everything else.</p>
<div class="pst-scrollable-table-container"><table class="table">
<tr class="row-odd"><th class="head"><p>backend</p></th>
<th class="head"><p>manager.resize(w,h)</p></th>
<th class="head"><p>Extra</p></th>
<tr class="row-even"><td><p>gtk3</p></td>
<td><p>window</p></td>
<tr class="row-odd"><td><p>Tk</p></td>
<td><p>canvas</p></td>
<tr class="row-even"><td><p>Qt</p></td>
<td><p>window</p></td>
<tr class="row-odd"><td><p>Wx</p></td>
<td><p>canvas</p></td>
<td><p>FigureManagerWx had
<code class="docutils literal notranslate"><span class="pre">frame</span></code> as an alias to
window, so this also
breaks BC.</p></td>
<section id="alternatives">
<h2><a class="toc-backref" href="#id8" role="doc-backlink">Alternatives</a><a class="headerlink" href="#alternatives" title="Link to this heading">#</a></h2>
<p>If there were any alternative solutions to solving the same problem,
they should be discussed here, along with a justification for the
chosen approach.</p>
<section id="questions">
<h2><a class="toc-backref" href="#id9" role="doc-backlink">Questions</a><a class="headerlink" href="#questions" title="Link to this heading">#</a></h2>
<p>Mdehoon: Can you elaborate on how to run multiple backends
concurrently?</p>
<p>OceanWolf: &#64;mdehoon, as I say, not for this MEP, but I see this MEP
opens it up as a future possibility.  Basically the <code class="docutils literal notranslate"><span class="pre">MainLoopBase</span></code>
class acts a per backend Gcf, in this MEP it tracks the number of
figures open per backend, and manages the mainloops for those
backends.  It closes the backend specific mainloop when it detects
that no figures remain open for that backend.  Because of this I
imagine that with only a small amount of tweaking that we can do
full-multi-backend matplotlib.  No idea yet why one would want to, but
I leave the possibility there in MainLoopBase.  With all the
backend-code specifics refactored out of <code class="docutils literal notranslate"><span class="pre">FigureManager</span></code> also aids
in this, one manager to rule them (the backends) all.</p>
<p>Mdehoon: &#64;OceanWolf, OK, thanks for the explanation. Having a uniform
API for the backends is very important for the maintainability of
matplotlib. I think this MEP is a step in the right direction.</p>
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